Changes in fluorescence induced by infrared multiphoton excitation of optically excited SO2

Abstract

Photophysical properties of excited SO2 were studied by the UV+MPE technique: single-photon UV excitation together with IR multiphoton excitation (MPE). Compared to the fluorescence in the absence of IR irradiation, the fluorescence following MPE showed a decrease of the main fluorescence peaks, accompanied by the appearance of broad fluorescence features to the high-energy side of the excitation frequency. Different CO2 laser lines were about equally effective. The dependence on laser fluence can be accounted for quantitatively using a simple stepladder model for MPE. The cross section for absorption of CO2 laser radiation by SO2, excited to around 32 000 cm−1, was found to be σ = 1.5×10−21 cm2. The effective decay time of the fluorescence with IR-on is increased twofold, compared to that with IR-off. This increase seems to parallel the reported dependence of the lifetimes on excitation frequency. It may be interpreted as due to more efficient coupling of excited vibronic states with highly-excited vibrational states in the ground electronic state.